It is well-known that fruit and vegetables are an important source of micronutrients such as vitamins and minerals. Heat processing of vegetables or fruit, as is commonly applied in the food industry, can have a pronounced impact on the levels of these vitamins in the final product as well as on their oral bioavailability. Here the term “oral bioavailability” refers to the fraction of the total amount of an ingested vitamin that ultimately reaches the systemic circulation. Thus, an oral bioavailability of 20% means that only 20% of the ingested amount of vitamin reaches the systemic circulation.
Processing of plant materials, including heat processing, can cause a reduction in vitamin content as a result of e.g. enzymatic or heat-induced degradation. Vitamin C and folates (vitamin B11) are examples of nutritionally important hydrophilic vitamins that are sensitive to enzymatic and heat-induced oxidative degradation. Heat processing of plant materials can also adversely affect the content of hydrophilic vitamins if such heat processing utilizes large quantities of water and if this water is discarded together with the dissolved vitamins contained therein.
Heat processing can also have a favorable impact on vitamin content, as it can be used to inactivate endogenous vitamin-degrading enzymes that become active when plant materials are cut or ground. In addition, heat processing can have a positive effect on the oral bioavailability of especially lipophilic (pro)vitamins, such as β-carotene. It has been shown, for instance, that oral bioavailability of some vitamins in heat processed fruit or vegetables is substantially higher than in the fresh fruit or vegetables.
The oral bioavailability of a vitamin that is contained within a fruit or vegetable is determined by three successive processes. First the vitamin must be released from the vegetable or fruit matrix into the juices of the gastrointestinal tract. The extent to which a vitamin is released from a vegetable or fruit into the gastrointestinal tract determines its so called bioaccessibility. In the case of hydrophobic compounds, partitioning into dietary mixed micelles is required for effective absorption. Secondly, the released nutrient must be transported across the intestinal epithelium into the portal vein and/or it must be transported in chylomicrons via the lymph into the bloodstream before it enters the heart (absorption). Finally, in order to reach the systemic circulation the vitamin that has been transported into the portal vein must pass the liver without being metabolized. Consequently, oral bioavailability of a vitamin is determined by its bioaccessibility, its absorption and its metabolization.
In the case of vegetables and fruit, limited bioavailability of lipophilic vitamins is mainly associated with limited bioaccessibility and/or limited absorption.
Heat processing of blends of different types of fresh fruit or fresh vegetables offers a special challenge, especially if these blends are composed of fruit/vegetables containing heat-sensitive vitamins as well as fruit/vegetables containing vitamins whose oral bioavailability is low. In general, it is impossible to find heating conditions that minimize degradation of heat-sensitive vitamins whilst at the same time maximizing the oral bioavailability of the vitamins contained within the blend. Hence, there is a need for a procedure for heat processing blends of fresh fruit or vegetables that maximizes the content as well as oral bioavailability of different vitamins, notably lipophilic as well as hydrophilic vitamins.
Miglio et al. (Effects of Different Cooking Methods on Nutritional and Physicochemical Characteristics of Selected Vegetables, J. Agric. Food Chem. (2008) 56, 139-147) describe the results of a study that investigated the effect of three common cooking practices (boiling, steaming and frying) on phytochemical contents (polyphenols, carotenoids, glucosinolates, and ascorbic acid). The authors state that an overall increase of Trolox equivalent antioxidant capacity (TEAC), ferric reducing antioxidant powder (FRAP) and total radical-trapping antioxidant parameter (TRAP) was observed in all cooked vegetables, probably because of matrix softening and increased extractability of compounds., which could be partially converted into more antioxidant chemical species. In addition, the authors conclude that their finding defy the notion that processed vegetables offer lower nutritional quality and also suggest that for each vegetable method a cooking method would be preferred to preserve the nutritional and physicochemical qualities.
Parada et al. (Food Microstructure Affects the Bioavailability of Several Nutrients, J. of Food Sc. (2007) 72(2), 21-32) provide a review summarizing relevant in vivo and in vitro methods used to assess the bioavailability of some nutrients (mostly phhytochemicals), types of microstructural changes imparted by processing and during food ingestion that are relevant in matrix-nutrient interactions, and their effect on bioavailability of selected nutrients.